The present invention relates to a method for preparing optically active acids and their corresponding esters by partial enzymatic hydrolysis of .alpha.-tertiary carboxylic acid esters using enzymes classified as serine carboxypeptidases or structurally and catalytically homologous enzymes. Such enzymes are generally classified by the Enzyme Commission in class EC 3.4.16.1. The method of this invention is useful in preparing compounds which have utility as starting materials and intermediates for the synthesis of pharmaceutical, agricultural and veterinary products.
Review articles on enzymatic synthesis, such as D. H. G. Crout and M. Christen, Modern Synthetic Methods, vol. 5 (1989 , Springer-Verlag and J. B. Jones, Tetrahedron, 42, 3351 (1986), describe many examples of the enzymatic resolution of esters in which the .alpha.-carbon possesses one hydrogen atom. However, relatively little information has been reported for the enzymatic resolution of carboxylic acids or esters in which the .alpha.-carbon is fully substituted with moleties other than hydrogen.
Chenault et al., J. Am.. Chem. Soc., 111, 6354 (1989), describe the resolution of eight 2-amino-2-methyl carboxylic acids by partial hydrolysis of the amide moleties of racemic 2-N-acylamino-2-methyl carboxylic acids with acylase I from porcine kidney and from the fungus Aspergillus species. The enzymes display (S)-stereoselectivity, and three of the L-2-methylamino acids were prepared in greater than 90% enantiomeric excess. Results of the resolution of the N-acylmethyldopa derivative were not disclosed. The reported enzymatic resolutions require as much as twice the weight of enzyme to substrate and about eleven days to achieve 50% hydrolysis, and are therefore limited in their applications and in their usefulness in commercial processes.
Sugai et al., J. Org. Chem. 55, 4643 (1990) describe the resolution of the racemic methyl ester of 2-benzyloxy-2-methyl-4-pentenoic acid (an .alpha.-oxygen substituted ester) to yield the optically pure (S)-acid which was subsequently used in the synthesis of (1S,5R)-(-)-frontalin, a constituent of the aggregation pheromone of the female southern pine bark beetle. The enantiomerically pure {S)-acid was obtained by enzymatically hydrolysing the racemic ester with Candida cylindracea lipase, isolating and then re-esterifying the enantiomerically enriched (S)-acid, and recontacting the optically enriched (S)-ester with Candida cylindracea lipase for additional hydrolysis. This sequence for preparing (S)-2-benzyloxy-2-methyl-4-pentenoic acid that requires two enzymatic resolution steps results in longer overall reaction times and lower chemical yields than a process which employs only one enzymatic resolution step. Enzymatic resolution of .alpha.-tertiary 2-hydrazino-2-methyl carboxylic acid esters has not previously been reported in the literature. Such compounds are useful in the production of amino acid decarboxylase inhibitors, and in particular carbidopa, which is used in the treatment of Parkinson's disease. The current methods of production of compounds such as carbidopa employ chemical resolution of 2-hydrazino (U.S. Pat. No. 3,895,052) or 2-amino (U.S. Pat. No. 3,405,159) precursors. Such methods require expensive chiral auxiliaries or costly and complicated equipment. See also U.S. Pat. No. 3,830,827, which describes a chemical method for preparing carbidopa.